Carbon cloth annealing process



1967 J. B. GALLAGHER 3,304,148

CARBON CLOTH ANNEALING PROCESS Filed June 17, 1963 CLOTH r HEAT TO 800F INERT ATMOSPHERE ACID WASH HEAT TO IOOO 2000 F INERT ATMOSPHERE COOL HEA'fTo lOOO-2000 E m PRESENCE OFOXYGEN INVENTOR (Tam E filing 222 224M flag 42AM ATTORNEYS United States Patent M 3,304,148 CARBON CLOTH ANNEALING PROCESS John B. Gallagher, Newark, Del., assignor, by mesne assignments, to Haveg Industries, Inc., a wholly owned subsidiary of Hercules Powder Company, New Castle,

Del., a corporation of Delaware Filed June 17, 1963, Ser. No. 288,085 11 Claims. (Cl. 8-140) This invention relates to the preparation of carbon fibers.

It has been proposed in the past to prepare carbon fibers from cellulosic materials by heating the same in an inert atmosphere, e.g., Soltes Patent 3,011,981 and Abbott Patent 3,053,775. However, it has been found difficult to obtain reproducible, good quality, high strength carbon fibers, e.g., in the form of yarn or cloth, by present procedures. While good quality products are obtained in some runs, in other runs employing exactly the same conditions the quality is poor. More recently it has been proposed to improve such procedures either by washing regenerated cellulose with a volatile organic solvent prior to heating in an inert or non-oxidizing atmosphere to carbonization temperature (Ohsol et al. application Serial No. 240,296, filed November 27, 1962) or by water washing regenerated cellulose fibers having a sodium content above 25 ppm. until the sodium content is below 25 ppm. and then heating the regenerated cellulose in an inert or non-oxidizing atmosphere to carbonization temperature (Ohsol application Serial No, 230,214, filed October 12, 1962). The procedures of Ohsol et al. and Ohsol give improved results, particularly on small scale runs. However, in some instances in large scale runs the uniformity in improvement in burst strength is not as great as desired.

While not being limited to any theory it appears that in normal pyrolysis of carbon fibers in a completely inert atmosphere, a certain amount of highly cross-linked brittle resinous material appears to be deposited on the surface of the carbon fibers. Possibly, this is produced by recombination of some of the products of pyrolysis of the original rayon. This surface resin, being brittle, causes the whole structure to be brittle and to exhibit very low burst and tear strength.

Accordingly, it is an object of the present invention to prepare good quality carbon fibers from regenerated cellulose, e.g., in the form of yarn, cloth and the like, with reproducible results.

Another object is to prepare carbon fibers from regenerated cellulose in the form of yarn and cloth having increased burst and tear strength.

A further object is to devise an improved procedure for forming carbon fibers from regenerated cellulose.

Yet another object is to improve the strength of carbon cloth produced by normal methods, viz., pyrolization of rayon under an inert atmosphere.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by Way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

It has now been found these objects can be obtained if carbon fibers, preferably, in the form of cloth, made by carbonizing cellulose (preferably regenerated cellulose), in an inert or non-oxidizing atmosphere at an elevated temperature is subsequently annealed in an oxygen-containing atmosphere at an elevated temperature. The annealing is usually carried out at a temperature of 1000 3,304,148 Patented Feb. 14, 1967 to 2000 F., although the temperature can be somewhat lower, e.g., 800 F. or can be above 2000 F. The time of treatment is usually quite brief, e.g., 5 seconds to 5 minutes. Longer periods of time can be used, but there is some loss of product due to the oxidation of the carbon fibers to carbon monoxide and/ or carbon dioxide.

The amount of oxygen in the oxygen-containing atmosphere can range from 5% to The balance, if any, can be an inert gas, such as nitrogen, argon, carbon dioxide, helium or nitrogen dioxide. In general, the higher the oxygen content of the gas, the shorter the time required. However, the greater the oxygen content of the gas, the more carefully is it required to control the treatment to avoid excessive burning of the carbon. The preferred oxygen-containing gas is air. There can be used ozone alone or mixed with air or an inert gas as the source of oxygen. The use of corona discharge in air which forms ozone can also be employed. The voltage can be 10,000 volts or higher, e.g., 50,000 volts. The surface of the carbon fibers is heated by the corona discharge operation.

The carbonized cloth to be treated according to the invention can be cooled after the carbonizing (pyrolysis) treatment in an inert atmosphere and then reheated briefly in the presence of an oxygen-containing gas to anneal the same. Alternatively, after the carbonizing treatment in an inert atmosphere at elevated temperature oxygencontaining gas is bled into the atmosphere and the annealing treatment is carried out. Thus, 10% of air can be bled into the inert gas toward the end of the pyrolysis procedure. Yet another alternative procedure is to reheat the carbonized cloth in an inert atmosphere and allow the cloth to cool in the presence of air or other oxygencon-taining gas.

The process of the present invention burns off the relatively reactive surface layer of resin and exposes the stronger and more flexible fibrous material underneath. The material then exhibits its normally good tear resistance and burst strength in uniform fashion. There has been observed a -15 to 25% weight loss during the annealing operation.

The process of the invention burns off the short whiskers which normally are present on the carbonized cloth.

The process of the present invention has been found to materially increase the burst and tear strength of carbonized cloth. It is also useful in upgrading production material that falls below strength specifications.

The process of the present invention can be employed with carbonized fibers, preferably cloth, produced by any of the known processes employing an inert atmosphere for pyrolysis. Thus, there can be employed carbon fibers, e.g., cloth, produced in the manner set forth in the aforementioned Soltes patent, Abbott patent, Ohsol application Serial No. 230,214 or Ohsol et al. application Serial No. 240,296.

As the starting cellulosic material there is preferably employed regenerated cellulose. As used in the present specification and claims the term regenerated cellulose is intended to embrace fibers of viscose rayon, cuprammonium rayon and saponified cellulose ester r-ayon, e.g., saponified cellulose acetate rayon. The cellulose is heated in a non-oxidizing atmosphere. As the non-oxidizing atmosphere there can be used inert gases such as nitrogen, helium, argon, or even a vacuum, e.g., 0.001 mm. or 2 mm. Also, there can be employed reducing gases such as hydrogen and there can be used ammonia.

The heating to pyrolyze the cellulose is usually carried out in two stages. The first stage is a gradual heating up to 800 F. and the second stage is a heating from 800 F. to 1500 F. or above. The partially carbonized fibers can be cooled between the first and second stages but while this is preferred, it is not essential. If cooling is employed between the first and second stages there is preferably included an acid wash at a pH of 1-5, e.g., with a 1 to 10% aqueous solution of an organic or inorganic acid such as acetic acid, propionic acid, formic acid, oxalic acid, hydrochloric acid, phosphoric acid or sulfuric acid. Preferably, acetic acid or hydrochloric acid is employed.

The preferred method of pyrolysis is to heat dried regenerated cellulose in the non-oxidizing atmosphere, e.g., in less than 2 minutes, to a temperature of 250 to 400 F., preferably between 300 and 400 F. It is then heated slowly at a temperature of 250 to 500 F., most preferably, between 300 and 500 F. While the time can be as short as 2 hours, it is preferably at least 12 hours and can be as long as a week. The product is then gradually heated at a temperature of from 500 to 800 F. for 2 to 24 hours or longer, usually for 4 to 12 hours. Next, the temperature, with or without intermediate cooling to 100 F. or below, is raised from 800 to 1500 F., or above, over a period of 2 to 1800 hours. At a top temperature of 1500 F. the product has 94% carbon, at a top temperature of 1800 F. the product has 96.5 to 97.5% carbon, and at a top temperature of 2000 F. the product contains 98 to 99% carbon. If the final temperature is elevated to 3500 F. or above,.e.g., 5000 F., the product is converted into graphite fibers or cloth.

The carbonized fibers, e.g., in the form of yarn or cloth, are then given the treatment with an oxygen-containing gas at an elevated temperature according to the present invention.

The single figure of the accompanying drawing is a diagrammatic illustration of one preferred form of the present invention.

Unless otherwise indicated, all parts and percentages are by weight.

Using ordinary viscose rayon of square weave and with a weight of about 18 oz./sq. yd. as starting material, the final carbon bloth treated according to the present invention weighed about 7 oz./ sq. yd.

Example 1 136 yards of viscose rayon cloth having 3400 filaments in the yarn and having 20 x 20 threads per square inch were placed in a retort which had previously been purged with nitrogen gas. The retort having a nitrogen atmosphere was gradually heated to 450 F. and held for 1 hour. The retort temperature was then brought up to 500 F. and held for 2 hours. The retort temperature was then brought up to 600 and held for 1% hours. The retort temperature was then brought up to 650 F. and held for 1 hour. The retort temperature was then brought up to 700 F. and held for one-half hour. The retort temperature was then brought up to 750 to 770 F. and held for three-fourths of an hour. The oven was then turned off and the retort was cooled to 100 F. over a 6 hour period and then opened.

The material at this point had no brittle spots, had very good hand, and exhibited Mullen burst strength of 100 to 120 p.s.i.g.

After the cloth was removed from the retort, it was immersed in 5% aqueous hydrochloric acid at room temperature (70 F.) for 6 hours, removed, water rinsed and dried.

40 yards of the material from the acid treatment was loaded into a retort which had been purged with nitrogen to give an inert atmosphere. The retort temperature was brought up to 500 F. and held there and at succeeding 100 F. levels for 1 hour intervals until 1000 F. was reached. The retort temperature was then raised on the average of 100 F. every one-half hour until a retort temperature of 1680 F. was reached. The furnace was then turned off and the retort was cooled to 100 F. over a 10 hour period and then opened.

The cloth at this point exhibited a somewhat harsh hand and had a Mullen burst strength of to 25 p.s.i.g.

This carbon cloth was then heated to 1450" F. for approximately 1 minute in the presence of air and cooled to room temperature. The average burst strength of the treated cloth increased to 50 to 60 p.s.i.g. and the cloth had a good hand.

Another portion of the cloth was heated to 1500 F. for 1 minute and quenched in room temperature air. The treated product had a burst strength of 60 p.s.i.g.

Example 2 A carbon cloth was made by pyrolysis as in Example 1 but omitting the acid wash and cooling between the first and second pyrolysis stages. The cloth had good hand but was weak with a Mullen burst strength of 10 to 20 p.s.i.g. A Bernz-O-Matic propane torch in the room atmosphere, i.e., air, was applied to an area about 1 inch in daimeter while the torch was held about 1 inch away from the cloth until the 1 inch circle glowed cherry red. Upon removing the flame from the cloth, the cloth stopped glowing and quickly lost its heat. The strength of the treated area was tested by probing and tugging and found to be much stronger than before. It was noticed that when the flame was first directed at the cloth the short whiskers on the cloth in the immediate vicinity of the flame were burned off. The Mullen burst strength of the annealed area of the cloth was measured and the increased strength first observed by the tugging was borne out by a burst reading of 50-60 p.s.i.g. The flame temperature of the propane torch was measured and found to be 1670 F.

Another sample of the carbon cloth about 12" x 12" was placed in a furnace at 1450 F. and then allowed to cool to room temperature in air. The Mullen burst strength was found to increase to 50 to 60 p.s.i.g. as in the previous test.

Example 3 A piece of carbon cloth made by pyrolyzing hydrolyzed cellulose acetate rayon at a temperature which ranged up to 1500 F. and which was brittle to the touch and had a burst strength of 2 to 5 p.s.i.g. was heated to 1450 F. for 1 minute in the presence of air and allowed to cool to room temperature. Its burst strength was increased to 50 to 60 p.s.i.g., and its brittleness was removed.

I claim:

1. A process of improving the burst strength of carbon fibers which had been prepared by carbonizing cellulose fibers in a non-oxidizing atmosphere at elevated temperatures, said process comprising treating the carbon fibers at a temperature of at least 800 F. in the presence of an atmosphere containing at least 5% oxygen.

2. A process of improving the burst strength of carbon cloth which had been prepared by carbonizing regenerated cellulose cloth in a non-oxidizing atmosphere at elevated temperatures, said process comprising treating the carbon cloth at a temperature of 800 F. to 2000 F. in the presence of an atmosphere containing at least 5% oxygen.

3. A process according to claim 2 wherein the oxygen containing atmosphere is air.

4. A process of improving the burst strength of carbon cloth which had been prepared by carbonizing regenerated cellulose cloth in a non-oxidizing atmosphere at elevated temperatures comprising treating the cloth with an atmosphere of air at 1000 to 2000 F.

5. A process of improving the burst strength of carbon cloth which has short whiskers thereon and which had been prepared by carbonizing regenerated cellulose cloth in a non-oxidizing atmosphere at elevated temperature comprising burning 01f the whiskers at a temperature of at least 800 F. in an atmosphere containing at least 5% oxygen.

6. A process according to claim 5 wherein the oxygencontaining atmosphere is air and the burning is done at a temperature of between 800 and 2000 F.

7. A process according to claim 1 wherein the carbon cloth is subjected to a temperature of at least 1000 F. and is allowed to cool in the presence of a gas containing at least 5% oxygen.

8. A process comprising carbonizing cellulosic fibers at a gradually increasing temperature in a non-oxidizing atmosphere, said temperature reaching at least 1500 F. and thereafter subjecting the carbonized fibers to a gas containing at least 5% of oxygen at a high temperature of 800 F. to 2000 F.

9. A process according to claim 8 wherein the cellulosic fibers are in the form of regenerated cellulose cloth.

10. A process according to claim 9 wherein the oxygen containing gas is air.

11. A process according to claim 10 wherein the high temperature is 1000 to 2000 F. and the time of treatment with air is 5 seconds to 5 minutes.

References Cited by the Examiner UNITED STATES PATENTS NORMAN G. TORCHIN, Primary Examiner.

J. RAUBITSCHEK, Assistant Examiner. 

1. A PROCESS OF IMPROVING THE BURST STRENGTH OF CARBON FIBERS WHICH HAD BEEN PREPARED BY CARBONIZING CELLULOSE FIBERS IN A NON-OXIDIZING ATMOSPHERE AT ELEVATED TEMPERATURES, SAID PROCESS COOMPRISING TREATING THE CARBON FIBERS AT A TEMPERATURE OF AT LEAST 800*F. IN THE PRESENCE OF AN ATMOSPHERE CONTAINING AT LEAST 5% OXYGEN. 